The letter, written by Harbottle & Lewis, was written after the law firm had examined emails from six individuals, including the paper’s former editor Andy Coulson and Clive Goodman, the former royal reporter jailed for phone hacking in 2007.

It was used by News International to defend against claims of widespread wrongdoing by claiming that there was no “reasonable evidence that…illegal action’s were known about” by Mr Coulson.

Last month, however, the former Director of Public Prosecutions Lord Macdonald said that when he looked at the emails he spotted evidence of criminal activity “within minutes”.

When the emails were handed to Scotland Yard in June, police launched a new investigation into allegations that the newspaper had made illegal payments to police officers in exchange for information.

Now it has been claimed that the letter was re-written in language which, while claiming that Mr Coulson did not know about phone hacking, was sufficiently vague as to whether there was evidence of other criminality.

The claims were made in The New York Times this weekend. A source is said to have told the paper that early drafts of the letter were rejected by News International as they were not broad enough.

The source said: “They [NI] wanted to bury those emails and they wanted Harbottle & Lewis to give them a letter to indicate there was nothing incriminating in the file."

The final wording of the letter from Harbottle & Lewis to News International read: “I can confirm that we did not find anything in those emails which appeared to us to be reasonable evidence that Clive Goodman’s illegal actions were known about and supported by both or either of Andy Coulson, the editor, and Neil Wallis, the deputy editor and/or that Ian Edmondson, the news editor, and others were carrying out similar procedures.”

The emails allegedly show discussions between Mr Coulson and Goodman, both of whom have been arrested by police investigating the alleged payments to police.

According to The New York Times the conversation shows Goodman complaining about a cutback on cash payment to sources, saying that he needed to pay his contacts in Scotland Yard’s royal protection unit.

In another Goodman says he does not want to go into details about cash payments because everyone involved “could go to prison for this.”

The emails allegedly include a request from Goodman to Mr Coulson for £1,000 to buy the Green Book, a directory which hold the personal telephone numbers of members of the royal family.

A spokesman for Harbottle & Lewis said that the firm could not make any public comment on the allegations except to the police and to parliamentary committees.

The spokesman confirmed that the firm would speak with police in the coming days.